SCRs in AC Circuits

      In the circuit shown in Figure 5-1, the gate of the SCR is connected through a resistor and diode directly to its anode. When the AC voltage applied to the anode rises in the positive direction, current flows through the gate-cathode section of the SCR. When it reaches the trigger point (assume 5 milliamperes for this SCR), the SCR fires and conducts through the anode-cathode section. It conducts as long as the AC voltage remains in positive direction and the current is above its holding current level.

      When the AC voltage drops to zero and begins to increase in the negative direction, the SCR remains turned off. When the voltage applied to anode again becomes positive the gate will trigger the SCR on again. The output of SCR is half-wave rectified direct current.

       Why use an SCR if it does the same thing as a simple junction diode? The SCR can be controlled as to when it turns on, Figure 5-2. A variable resistor has been added to the gate circuit of the figure. Assume that the gate current must reach a 5 mA level before the SCR will fire. By adjusting the variable resistor, it is possible to determine how much voltage must be applied to gate before a 5 mA current flows through the gate circuit. By adjusting the resistor to a higher value,it is possible to keep the SCR from firing until the AC voltage has reached its peak value in the positive direction. In this way the SCR will fire when the AC voltage is at its positive peak. With this setting of the resistor, the SCR will drop half the voltage and the load will drop the other half.

      By reducing the resistance of gate-circuit, the gate current reaches 5 mA sooner, and the SCR fires earlier in the AC cycle. This causes less voltage to be dropped across the SCR and more to be dropped across the load.

       If the gate resistance is reduced even more, the 5mA of gate current will be reached even sooner during the cycle and the SCR will fire earlier. Still less voltage is dropped across the SCR and more voltage is dropped across the load.

       There is a problem with this type of control. The SCR controls half of the positive cycle of alternating current applied to it. The latest that the SCR can be fired is when the positive half cycle has reached 90°. This permits the SCR to control only half of the AC positive wave; half voltage is applied to the load when the SCR initially fires. If the load resistor is a light bulb, the bulb will burn at half brightness when it is first turned on.

      在圖5-1中所示的電路中，可控硅的柵極通過一個電阻器和二極管直接將其陽極連接。當施加到陽極的交流電壓上升，在正方向上，電流流經可控硅的柵極 - 陰極部分。當達到觸發點（假定為5毫安這個SCR） ，可控硅觸發，并進行通過陽極 - 陰極部分。它進行，只要在交流電壓保持在正方向和電流高于其保持電流電平。

      當AC電壓下降到零，并開始增加在負方向上，這個SCR仍然關閉。當施加到陽極的電壓再次變為正的柵極將再次觸發可控硅上。可控硅的輸出進行半波整流的直流電流。

       所以，如果做同樣的事情使用的SCR作為一個簡單的結型二極管？可控硅可以當它打開時，圖5-2進行控制，以。一種可變電阻器已被添加到圖中的門電路。假設柵極電流必須達到5毫安級之前， SCR將閃光。通過調節可變電阻器，它能夠確定多少電壓如何，必須施加到柵極5 mA的電流流過柵極電路之前。通過調節電阻為較高的值，因此能夠保持在SCR從擊發直到AC電壓達到其峰值的正方向。通過這種方式， SCR將火當交流電壓為正峰值。與電阻器的此設置中，SCR將下降一半的電壓和負載就會下降的另一半。

      通過減小柵極電路的電阻，柵極電流達到5毫安越早，和SCR之前在AC周期閃光。這將導致較小的電壓被穿過SCR下降，更要在負載兩端丟棄。

       如果柵極電阻更是降低了，門極電流的5毫安將會更早的周期內達到和SCR將提早火。還是少電壓加在可控硅下降和更多電壓加在負載下降。

       存在與這種類型的控制的一個問題。可控硅控制的一半施加到其的交流電流的正周期。這個SCR可以發射最新的是當在正半周期已經達到了90℃ 。這使得SCR來控制交流正波的一半，另一半的電壓施加到在SCR初始觸發該負載。如果負載電阻是一個燈泡，燈泡會燃燒一半的亮度，當它第一次開啟。